191 1 ] A. J. ROSANOFF AND FLORENCE I. ORR 223 



The inheritance of any such character is beHeved to be depend- 

 ent upon the presence in the germ plasm of a unit of substance 

 called a determiner. 



With reference to any given character the condition in an indi- 

 vidual may be dominant or recessive: the character is dom inant 



when, depending upon the presence of its determiner in the germ 

 plasm, it is plainly manifest ; and it is recessive when, owing to 

 the lack of its determiner in the germ plasm, it is not present in 

 the individual under consideration. 



The dominant and recessive conditions of a character are desig- 

 nated by the symbols D and R respectively. 



Thus in the case of eye color the brown color is the dominant 

 condition and the blue color is the recessive condition. In other 

 words, the inheritance of brown eyes is due to the presence in the 

 germ plasm of a determiner upon which the formation of brown 

 pigment in the anterior layers of the irides depends, while the 

 inheritance of blue eyes is due to the lack of determiner for brown 

 pigment in the germ plasm, for the blue color of eyes is due 

 merely to the absence of brown pigment, the effect of blue being 

 produced by the choroid coat shining through the opalescent but 

 pigment-free anterior layers of the irides in such cases. 



It is obvious that as regards any character an individual may 

 inherit from both parents — duplex inheritance, designated by the 

 symbol DD, — or from one parent only — simplex inheritance, 

 designated by the symbol DR, — or he may fail to inherit from 

 either parent — nulliplex inheritance, designated by the symbol 

 RR; in the last case the individual will exhibit the recessive 

 condition. 



We are now in a position to estimate the relative number of 

 each type of offspring according to theoretical expectation in 

 the case of any combination of mates. 



There are but six theoretically possible combinations of mates. 

 Continuing to make use of eye color as an instance of a Men- 

 delian character, let us consider in turn each theoretical possi- 

 bility. 



I. Both parents blue-eyed (nulliplex) : all the children will be 

 blue-eyed, as may be shown by the following biological formula : 



RRxRRooRR. 



